1. Field of the Invention
The present invention relates to a rotational speed detector, and more particularly to a device for detecting a rotational speed and a rotational position of a rotating shaft of a supercharger compressor for used in an internal combustion engine.
2. Description of Related Art
A supercharger is used for compressing air to be supplied to an intake manifold of an internal combustion engine to obtain a higher output of the engine. A compressor of the supercharger is driven by various methods. For example, the compressor is rotated by a turbine driven by energy of exhaust gas. The compressor may be directly driven by a rotational torque of a crankshaft of an engine, or by an independent electric motor. The supercharger driven by the exhaust gas turbine (referred to as a turbocharger) may be assisted by an electric motor. This type of supercharger is referred to as a motor-assisted turbocharger.
In the supercharger, it is required to accurately detect a rotational speed and a rotational position of the compressor in order to adequately control operation of the supercharger. For example, in the turbocharger, an opening degree of a valve for introducing exhaust gas into a turbine is controlled based on a rotational speed of a compressor. In the motor-assisted turbocharger, a rotary electric machine is used as a motor for assisting the torque for driving the compressor when the engine is operating under a heavy load at a low speed. On the other hand, when the engine is operating under a light load at a high speed, the rotary electric machine is used as a generator for charging an on-board battery. It is also possible to drive another motor for assisting the engine by the electric power stored in the on-board battery. The rotary electric machine used in the motor-assisted turbocharger is switched to a motor or a generator according to a rotational speed and a rotational position of the compressor.
To detect a rotational speed and a rotational position of a compressor in a motor-assisted turbocharger, JP-A-5-79340 proposes a device in which a permanent magnet is embedded in a compressor blade and magnetic poles are disposed in a stationary member facing the compressor blade. The permanent magnet is embedded in the compressor blade so that its longitudinal direction extends in the traveling direction of the blade and the longitudinal length (a distance between poles of the permanent magnet) coincides with a distance between the magnetic poles disposed in the stationary member. The rotational speed and the rotational position of the compressor blade are detected based on magnetic flux that changes according to rotation of the compressor blade.
In the detecting device disclosed in JP-A-5-79340, however, the following problems are involved. Since the permanent magnet is embedded in the rotating compressor blade, a certain unbalance appears in the compressor blade. Further, a process of forming the compressor blade becomes complex because the permanent magnet has to be embedded in the compressor blade, making the manufacturing cost high.